All Temperature Numbers Temperature Listing

450-650C (842-1202F)

Dehydroxylation in kaolin, ball clay

Dehydroxylation is the release of structurally bound hydroxyl groups from clay minerals such as kaolinite and ball clays during firing. Unlike dehydration (which removes physically held water), dehydroxylation involves the breakdown of hydroxyl groups in the crystal lattice, producing water vapor and transforming the clay mineral into an amorphous phase known as metakaolin. In ceramic bodies, this reaction typically begins around 450°C and is largely complete by about 600–650°C. In the manufacture of calcined kaolin, the process is considered essentially complete around this temperature. While some overlap exists between the loss of adsorbed water and the beginning of structural hydroxyl loss, it occurs well above the temperatures where free pore water is removed. Because the clay body has already become porous by this stage and the reaction proceeds gradually, dehydroxylation does not generate steam pressure problems.

Related Information

When kilns are not candled long enough

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Candling of kilns is the final stage of drying. Driers cannot achieve the temperatures needed to remove all water, so almost all industries rely on early stages of firing to remove it fully. Failures like this are part of the learning-curve of every company (because there is always pressure to fire as fast as possible).

Although much more common in heavy clay industries, porcelain insulators are one of the less likely products for this to happen with. This is because machine-forming methods make it possible to use aluminous porcelain bodies having very little clay. Thus, faster drying (with less shrinkage and fewer residual internal stresses) also makes it possible for early stages of firing to be quicker. But there are limits. These insulators are solid, thick and heavy. And they have extreme variations in thickness (the skirts and spindle). So, for even these, early stages of firing must be conducted carefully. For such products, periodic firings of days is often needed.

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